JP4403286B2 - Cemented carbide tool material and manufacturing method thereof - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a cemented carbide tool material to which carbon nanotubes are added, and a method for producing the same, and a material with higher toughness and hardness, such as a cutting tool, wear resistance, and impact resistance tool, is desired. The field related to the provision of metal materials for cutting processing belongs to the field of materials such as dies that are more efficient and long-life using the material.

  As conventional high-hardness alloy tool materials, WC-Co-based cemented carbide alloys, dispersed hard-type alloys typified by cermets such as TiC-Ti-based and WC-Ti-based are widely used. The WC-Co cemented carbide has a hardness of 90.5 to 92.5 (HRA), a bending strength of 1400 to 2500 (MPa), a Young's modulus of 450 to 600 (GPa), and a cermet having a hardness of 90 to 92.7 ( HRA) and bending strength of about 1700 to 2100 (MPa).

Cemented carbide or cermet is a composite material obtained by sintering a hard phase composed of carbides or carbonitrides of group 4a, 5a, or 6a of the periodic table with an iron-based metal such as Fe, Co, or Ni as a binder. It is. A material having the best mechanical properties is a WC-Co-based alloy, and this alloy is called a cemented carbide in a narrow sense. Each alloy is characterized by excellent low temperature hardness as well as high temperature hardness, high strength and stable physical properties. In addition to WC-Co cemented carbide, WC-Co-TiC-Co cemented carbide with improved oxidation resistance as a cutting tool, WC-Co-TaC-Co cemented carbide,
WC-TiC-TaC-Co based cemented carbide is used. On the other hand, the WC-Ni cemented carbide with Ni as the binder phase loses magnetism when the solid solution amount of W in Ni exceeds a certain level and becomes non-ferromagnetic, and the addition of Cr further improves corrosion resistance. WC-Ni-Cr cemented carbide has also been put into practical use.

As a means to increase the bending strength of cemented carbide, it is known to add a grain growth inhibiting component of WC during high-temperature sintering, and by this means, an ultrafine particle cemented carbide with high bending strength has been developed. (For example, Patent Document 1). Further, a material in which carbon nanotubes are dispersed in WC fine particles having a particle size of 0.54 μm or less and bulked by a pulse current heating sintering method has been developed as a cemented carbide for a Co-less mold (Non-patent Document 1). .
JP 2004-346370 A Nagano Prefectural Industrial Experiment Station Research Report No. 24-2004, p. 28-33

Although the wear resistance of cemented carbide has been further improved by the coating technique using the CVD (chemical vapor deposition) method, the basic rigidity and toughness of the material are not sufficient. As a tool material, particularly a cutting tool material, development of a material having the highest hardness of a cemented carbide and a toughness exceeding that of an ultrafine particle cemented carbide is required.

  The inventors have succeeded in developing a material having excellent characteristics as a tool material, particularly a cutting tool material, utilizing the physical characteristics of carbon nanotubes.

  That is, the cemented carbide tool material of the present invention is characterized in that the bending strength obtained by liquid phase sintering by adding 0.01 to 1.00% by weight of carbon nanotubes to the raw material powder is 2700 MPa or more. The tungsten carbide-cobalt cemented carbide tool material. This sintered cemented carbide tool material is a tungsten mill powder and a cobalt powder, and a powder in which carbon nanotubes are added so that the weight ratio in the total amount becomes 0.01 to 1.00% by weight is wet mixed by a ball mill. Then, it can be manufactured by liquid phase sintering at 1200 to 1500 ° C. in an inert gas atmosphere or in a vacuum.

  Cemented carbide materials are generally manufactured by powder metallurgy. For molding, a press molding method using a metal mold, a method of blending and kneading an organic binder with raw material powder to make a compound, which is fluidized by heating, injection molding or extrusion molding are used.

The formed body is usually pre-sintered at 600 to 1000 ° C. and then subjected to main sintering at a liquid phase appearance temperature or higher, usually about 1300 to 1550 ° C. As the temperature rises, W and C are dissolved in Co and solid phase sintering of the Co powder proceeds. At this time, if the amount of alloy carbon exceeds a specified value, free carbon is generated, and if the amount of alloy carbon is insufficient, η phase (Co ₃ W ₃ C) is generated and mechanical properties deteriorate.

  Therefore, the most important thing in the manufacture of cemented carbide is to obtain a healthy structure that does not produce η phase or free carbon. However, in a healthy structure in which no η phase or free carbon occurs, the W solubility in the Co liquid phase or the W solid solution amount in the binder phase at the solidification temperature varies depending on the amount of carbon alloy. Strict carbon control was required. For this reason, generally, a predetermined amount of carbon black is added to adjust the amount of carbon.

  In the present invention, the carbon nanotubes are dispersed and contained in the sintered cemented carbide structure, and by utilizing the physical characteristics of the carbon nanotubes, the bonding strength between the WC particles, which are ultrahard particles, and the Co metal of the binder is reduced. A strong cemented carbide tool material can be provided.

  The reason why the bending strength can be remarkably increased is not clear, but by adding carbon nanotubes to WC powder and Co powder and mixing them together and coexisting homogeneously, the toughness, which is the physical property of carbon nanotubes, is improved. It is presumed that it worked as a reinforcing material and resulted in significantly greater toughness.

  The bending strength of a conventional ultrafine cemented carbide is 2600-2700 MPa, and the present invention can provide a cemented carbide tool material having a bending strength of more than 2700 MPa, more preferably 3000 MPa or more. The tip of the cutting tool has a sharp edge and is easily chipped if it has poor toughness, but the tool life can be significantly extended by using the tool material of the present invention.

  Below, the manufacturing method of the tool material of this invention is demonstrated. The main raw material of the tool material of the present invention is tungsten carbide powder, and carbon nanotubes are added to and mixed with a mixture of cobalt metal powder as a binder.

  The carbon nanotubes used in the present invention are not limited to single-walled carbon nanotubes or multi-walled carbon nanotubes. Further, the diameter and length of the carbon nanotube are not limited, but from the viewpoint of ease of production and function development, the diameter of the carbon nanotube is preferably in the range of 1 to 20 nm and the length is preferably in the range of 1 to 20 μm. Preferably, single-walled carbon nanotubes having a diameter of 1 to 2 nm and a length of 1 to 2 μm, or multi-walled carbon nanotubes having a diameter of 1 to 20 nm and a length of 1 to 20 μm are used.

The apparent density of the carbon nanotube is about 0.78 to 1.8 (g / cm ³ ), and has the strength 100 times that of steel with the same weight. Although it does not react with acids and alkalis, has high corrosion resistance, and has a Young's modulus of about 400 to 500 GPa to more than 1000 GPa, in the present invention, it is preferable to use one having a Young's modulus exceeding 1000 GPa.

The average particle size of the WC powder is 0.5 to 12 μm, more preferably 0.5 to 1.0 μm, and the average particle size of the Co metal powder is 0.5 to 1.9 μm, more preferably 0.5 to 1. Use 0 μm. The average particle size is calculated according to the Fisher method JIS.
It is an average particle diameter measured with the apparatus by H2116. These powders and carbon nanotubes can be obtained as commercially available materials.

The proportion of the carbon nanotubes in the raw material powder mixture is preferably 0.01 to 1.00% by weight, more preferably 0.1 to 0.5% by weight based on the total amount. If the carbon nanotube powder is less than 0.01% by weight, physical properties cannot be exhibited. If the carbon nanotube powder exceeds 1.00% by weight, uniform dispersion cannot be achieved, and a good sintered body cannot be obtained. The Co powder of the binder in the raw material powder mixture is preferably 5 to 15% by weight. If the Co powder is less than 5% by weight, the bond strength is lowered, and if it exceeds 15% by weight, the hardness of the cemented carbide is lowered. More preferably, it is 8 to 12% by weight. In addition to the WC-Co cemented carbide, a conventionally known WC-TiC-Co cemented carbide,
A part of WC may be replaced with other hard particles such as WC-TaC-Co cemented carbide and WC-TiC-TaC-Co cemented carbide.

  The raw material powder is desirably wet pulverized and mixed by a ball mill using acetone, alcohol or the like as a solvent. It is desirable to use the physical properties of the carbon nanotubes and to mix well to join the hard particle interface. It is desirable that the mixed powder is put into a vacuum dryer and dried for 5 to 8 hours, taken out, and classified by a seam type meshler to obtain a powder having an average particle size of 0.4 to 1.5 μm.

  Next, the mixture is formed into a compact by a powder compression press or a cold isostatic press. Subsequently, the molded body is sintered at 1200 to 1500 ° C. If it is less than 1200 ° C., Co is not sufficiently liquid phase, and if it exceeds 1500 ° C., WC grows abnormally, which is inappropriate. More preferably, sintering is performed at 1350 to 1450 ° C. Temporary sintering can be performed at about 700 to 900 ° C. before this high-temperature sintering, and temporary molding and blank molding can be performed by cutting into a desired shape in an uncured state. If the pre-sintering temperature is less than 700 ° C., brittle fracture is likely to occur during the cutting process, and if it exceeds 900 ° C., hardness starts to appear and it becomes difficult to perform the cutting process.

Sintering must be performed in an inert gas atmosphere or in a vacuum. It is desirable to perform preliminary sintering at a degree of vacuum of about 10 ⁻² Torr and a degree of vacuum of about 10 ⁻³ to 10 ⁻⁵ Torr. In the atmosphere, the carbon nanotubes sublime at about 1200 ° C. In vacuum sintering, nitrogen gas is injected to prevent oxidation in the furnace, and vacuum vacuum absorbs impurities generated during sintering and controls the cooling rate from high temperature after sintering. It is desirable to do. In addition, a hot press, a hot isostatic press, and hot powder extrusion molding can also be used for sintering.

  Tungsten carbide (manufactured by Nippon Shin Metals, WC-F (C) particle size: 0.45 to 0.75 μm), cobalt (ULTRAFINE from umicore, particle size: 0.8 to 0.9 μm) and carbon nanotubes (Honjo Chemical: SWNTφ1-1. 2 nm, length 1-1.5 μm, Young's modulus 1060 GPa) was added, using cemented carbide balls (Φ6-9), wet (alcohol) grinding and mixing in a ball mill for about 72 hours, and this was put into a vacuum dryer Then, the mixture was dried for 5 to 8 hours, taken out, and classified by a seam type meshler, to prepare a powder having an average particle size of 0.5 μm having the composition ratio of Examples 1 to 3 shown in Table 1.

The uniformly mixed powder mixture is filled in a mold having a diameter of 80 mm and a depth of 50 mm, and pressed by a vertical powder compression press at a pressure of 1000 (kg / cm ² ) or by a cold isostatic press to 1000 ( A molded body having a predetermined size was formed by a pressing pressure of kg / cm ² This molded body is blank-molded in a batch-type vacuum furnace to be preliminarily sintered (temperature of 800 ° C.) to a predetermined dimension with an outer diameter thickness of 1 to 1.5 (mm), and then subjected to main sintering (temperature of 1380 ° C.). For 1 hour). In addition, as a comparative example, a cemented carbide with 0.14% by weight of carbon black added instead of carbon nanotubes was manufactured.

The obtained sintered body was precisely processed into a test piece size with a diamond grindstone of a surface grinder. The three-point bending strength of the test piece of the sintered body is, according to the old cemented carbide JIS B-4104, the test piece dimensions are width 8, thickness 4 (mm), distance between fulcrums 20 (mm), n = 5 It was. The hardness test was an AKASHI micro Vickers hardness tester with a test load of 1 (kg) and six locations. The three-point bending strength is a value without HIP treatment. Table 1 shows the obtained characteristics in comparison with comparative examples, conventional cemented carbide and cermet. The “CNT” column in the comparative example is the amount of carbon black added.
It can be seen that the cemented carbide of the present invention has significantly superior bending strength compared to the comparative examples, conventional cemented carbide and cermet.

  In FIG. 1, the scanning electron micrograph which observed the structure | tissue of the test piece is shown. FIG. 1 shows that the dispersion of the sintered raw material particles is very good. Moreover, the scanning electron micrograph which observed the torn surface after a 3 point | piece bending strength test in FIG. 2 is shown. FIG. 3 shows a scanning electron micrograph showing the fractured surface further enlarged.

The tool material of the present invention is conventionally used for cutting work in which a material with higher toughness and hardness is desired, such as a cutting tool in which cermet or cemented carbide is used, wear resistance, and impact resistant tool. The field related to the provision of metal materials is particularly useful, especially in the field of materials such as molds that are more efficient and long-life using the materials.

5 is a drawing-substituting scanning electron micrograph showing the structure of the alloy for the test piece obtained in Example 3. FIG. 5 is a drawing-substitute scanning electron micrograph showing a fracture surface after a three-point bending strength test for the test piece obtained in Example 3. FIG. 5 is a drawing-substituting scanning electron micrograph showing a test piece obtained in Example 3 with a broken surface further enlarged.

Claims (3)

The bending strength obtained by adding 0.01 to 1.00% by weight of carbon nanotubes to a raw material powder consisting of 85 to 95% by weight of tungsten carbide powder and the balance of cobalt powder and having a liquid phase sintering of 2700 MPa or more. A tungsten carbide-cobalt cemented carbide tool material. The sintered cemented carbide tool material according to claim 1, wherein the carbon nanotube is a single-walled carbon nanotube having a diameter of 1 to 2 nm, a length of 1 to 2 µm, and a Young's modulus of 1000 GPa or more. A powder in which carbon nanotubes are added to tungsten carbide powder and cobalt powder so that the weight ratio in the total amount becomes 0.01 to 1.00% by weight is wet-mixed by a ball mill, and then formed into an inert gas atmosphere. The method for producing a cemented carbide tool material according to claim 1, wherein liquid phase sintering is performed at 1200 to 1500 ° C. in a vacuum.